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🔗 Spaceflight 🔗 China

PAS-22, previously known as AsiaSat 3 and then HGS-1, was a geosynchronous communications satellite, which was salvaged from an unusable geosynchronous transfer orbit by means of the Moon's gravity.

🔗 Mathematics 🔗 Physics

Shor's algorithm is a polynomial-time quantum computer algorithm for integer factorization. Informally, it solves the following problem: Given an integer ${\displaystyle N}$, find its prime factors. It was invented in 1994 by the American mathematician Peter Shor.

On a quantum computer, to factor an integer ${\displaystyle N}$, Shor's algorithm runs in polynomial time (the time taken is polynomial in ${\displaystyle \log N}$, the size of the integer given as input). Specifically, it takes quantum gates of order ${\displaystyle O\!\left((\log N)^{2}(\log \log N)(\log \log \log N)\right)}$ using fast multiplication, thus demonstrating that the integer-factorization problem can be efficiently solved on a quantum computer and is consequently in the complexity class BQP. This is almost exponentially faster than the most efficient known classical factoring algorithm, the general number field sieve, which works in sub-exponential time — ${\displaystyle O\!\left(e^{1.9(\log N)^{1/3}(\log \log N)^{2/3}}\right)}$. The efficiency of Shor's algorithm is due to the efficiency of the quantum Fourier transform, and modular exponentiation by repeated squarings.

If a quantum computer with a sufficient number of qubits could operate without succumbing to quantum noise and other quantum-decoherence phenomena, then Shor's algorithm could be used to break public-key cryptography schemes, such as the widely used RSA scheme. RSA is based on the assumption that factoring large integers is computationally intractable. As far as is known, this assumption is valid for classical (non-quantum) computers; no classical algorithm is known that can factor integers in polynomial time. However, Shor's algorithm shows that factoring integers is efficient on an ideal quantum computer, so it may be feasible to defeat RSA by constructing a large quantum computer. It was also a powerful motivator for the design and construction of quantum computers, and for the study of new quantum-computer algorithms. It has also facilitated research on new cryptosystems that are secure from quantum computers, collectively called post-quantum cryptography.

In 2001, Shor's algorithm was demonstrated by a group at IBM, who factored ${\displaystyle 15}$ into ${\displaystyle 3\times 5}$, using an NMR implementation of a quantum computer with ${\displaystyle 7}$ qubits. After IBM's implementation, two independent groups implemented Shor's algorithm using photonic qubits, emphasizing that multi-qubit entanglement was observed when running the Shor's algorithm circuits. In 2012, the factorization of ${\displaystyle 15}$ was performed with solid-state qubits. Also, in 2012, the factorization of ${\displaystyle 21}$ was achieved, setting the record for the largest integer factored with Shor's algorithm.

🔗 Time 🔗 Moon

A blue moon is an additional full moon that appears in a subdivision of a year: either the third of four full moons in a season, or a second full moon in a month of the common calendar.

The phrase in modern usage has nothing to do with the actual color of the Moon, although a visually blue Moon (the Moon appearing with a bluish tinge) may occur under certain atmospheric conditions – for instance, if volcanic eruptions or fires release particles in the atmosphere of just the right size to preferentially scatter red light.

🔗 Mathematics 🔗 Biology 🔗 Physics 🔗 Plants

Tendril perversion, often referred to in context as simply perversion, is a geometric phenomenon found in helical structures such as plant tendrils, in which a helical structure forms that is divided into two sections of opposite chirality, with a transition between the two in the middle. A similar phenomenon can often be observed in kinked helical cables such as telephone handset cords.

The phenomenon was known to Charles Darwin, who wrote in 1865,

A tendril ... invariably becomes twisted in one part in one direction, and in another part in the opposite direction... This curious and symmetrical structure has been noticed by several botanists, but has not been sufficiently explained.

The term "tendril perversion" was coined by Goriely and Tabor in 1998 based on the word perversion found in the 19th Century science literature. "Perversion" is a transition from one chirality to another and was known to James Clerk Maxwell, who attributed it to the topologist J. B. Listing.

Tendril perversion can be viewed as an example of spontaneous symmetry breaking, in which the strained structure of the tendril adopts a configuration of minimum energy while preserving zero overall twist.

Tendril perversion has been studied both experimentally and theoretically. Gerbode et al. have made experimental studies of the coiling of cucumber tendrils. A detailed study of a simple model of the physics of tendril perversion was made by MacMillen and Goriely in the early 2000s. Liu et al. showed in 2014 that "the transition from a helical to a hemihelical shape, as well as the number of perversions, depends on the height to width ratio of the strip's cross-section."

Generalized tendril perversions were put forward by Silva et al., to include perversions that can be intrinsically produced in elastic filaments, leading to a multiplicity of geometries and dynamical properties.

🔗 Skepticism 🔗 Psychology

The Barnum effect, also called the Forer effect, or less commonly, the Barnum-Forer effect, is a common psychological phenomenon whereby individuals give high accuracy ratings to descriptions of their personality that supposedly are tailored specifically to them, that are in fact vague and general enough to apply to a wide range of people. This effect can provide a partial explanation for the widespread acceptance of some paranormal beliefs and practices, such as astrology, fortune telling, aura reading, and some types of personality tests.

These characterizations are often used by practitioners as a con-technique to convince victims that they are endowed with a paranormal gift. Because the assessment statements are so vague, people interpret their own meaning, thus the statement becomes "personal" to them. Also, individuals are more likely to accept negative assessments of themselves if they perceive the person presenting the assessment as a high-status professional.

The term "Barnum effect" was coined in 1956 by psychologist Paul Meehl in his essay Wanted – A Good Cookbook, because he relates the vague personality descriptions used in certain "pseudo-successful" psychological tests to those given by showman P. T. Barnum.

In the electronics industry, dark silicon is the amount of circuitry of an integrated circuit that cannot be powered-on at the nominal operating voltage for a given thermal design power (TDP) constraint. This is a challenge in the era of nanometer semiconductor nodes, where transistor scaling and voltage scaling are no longer in line with each other, resulting in the failure of Dennard scaling. This discontinuation of Dennard scaling has led to sharp increases in power densities that hamper powering-on all the transistors simultaneously at the nominal voltage, while keeping the chip temperature in the safe operating range. According to recent studies, researchers from different groups have projected that, at 8 nm technology nodes, the amount of Dark Silicon may reach up to 50–80% depending upon the processor architecture, cooling technology, and application workloads. Dark Silicon may be unavoidable even in server workloads with abundance of inherent client request-level parallelism.

🔗 Systems 🔗 Systems/Control theory

In control theory, a bang–bang controller (2 step or on–off controller), also known as a hysteresis controller, is a feedback controller that switches abruptly between two states. These controllers may be realized in terms of any element that provides hysteresis. They are often used to control a plant that accepts a binary input, for example a furnace that is either completely on or completely off. Most common residential thermostats are bang–bang controllers. The Heaviside step function in its discrete form is an example of a bang–bang control signal. Due to the discontinuous control signal, systems that include bang–bang controllers are variable structure systems, and bang–bang controllers are thus variable structure controllers.

🔗 California 🔗 Companies 🔗 Technology 🔗 Private Equity 🔗 California/San Francisco Bay Area

Y Combinator is an American seed accelerator launched in March 2005 and has been used to launch over 2,000 companies including Stripe, Airbnb, Cruise Automation, DoorDash, Coinbase, Instacart, and Dropbox. The combined valuation of the top YC companies was over $155 billion as of October, 2019.

🔗 Computer science

Go! is an agent-based programming language in the tradition of logic-based programming languages like Prolog. It was introduced in a 2003 paper by Francis McCabe and Keith Clark.